1. Field of the Invention
This invention relates to a method for improving the properties of active metal electrodes for use in nonaqueous electrochemical cells. More particularly, it relates to a process for the modification of conventional active metal electrodes by etching with an alcohol.
2. Description of the Prior Art
A substantial amount of interest has recently been centered on the development of ambient temperature, high energy density, electrochemical cells which are light in weight and capable of providing a higher voltage than conventional cells such as nickel-cadmium and lead-acid systems or alkaline cells having zinc anodes. The high energy density cell systems which are currently of interest typically involve the use of active metals (metals above hydrogen in the electromotive series of elements which are unstable in an aqueous environment) as anodes in combination with nonaqueous electrolytes. As used herein, "nonaqueous" is intended to mean substantially free of water.
In conventional electrochemical cells, cathode depolarizers are used in a form which will permit an intimate and maximum contact with an external electrical circuit, such as a set of wires connecting the electrodes of a cell, while also effecting a physical separation of the cathode depolarizer from the anode. In such cells, the cathode depolarizer is generally an insoluble, finely divided solid which is either admixed with or used as a coating over an inert conducting material, such as nickel, graphite or carbon rod, which serves as a current collector or cathode. The physical separation of the cathode depolarizer from the anode is necessary to prevent a direct chemical reaction between the anode material and the cathode depolarizer which would result in self-discharge of the cell.
Until recently, it was generally believed that a direct physical contact between the cathode depolarizer and the anode could not be permitted within an electrochemical cell. It has been discovered, however, that certain cathode depolarizers do not react chemically to any appreciable extent with active metal anodes at the interface between the anode and the cathode depolarizer. Accordingly, with materials of this type, it is possible to construct an electrochemical cell wherein an active metal anode is in direct contact with the cathode depolarizer. For example, U.S. Pat. No. 3,567,515 issued to Maricle et al. on Mar. 2, 1971, discloses the use of sulfur dioxide as a cathode depolarizer in such a cell. Similarly, U.S. Pat. No. 3,926,669 issued to Auborn on Dec. 16, 1975, discloses that certain liquid inorganic oxyhalides and thiohalides, such as thionyl chloride, sulfuryl chloride and phosphorus oxychloride, can be utilized as cathode depolarizers in such a cell.
Consistent with the disclosure of Maricle et al. in the above-mentioned U.S. Pat. No. 3,567,515, ultra-pure lithium electrodes prepared by vapor deposition of lithium on a glass substrate are stable when placed in direct contact with an electrolyte which comprises sulfur dioxide and in which the dithionite discharge product is soluble. However, we have found that a relatively rapid self-discharge usually occurs when the lithium electrode is fabricated from bulk samples of commercially supplied lithium. For example, when commercial lithium foil is placed in an electrolyte which comprises sulfur dioxide and in which the dithionite discharge product is soluble (dithionite anion is the sulfur dioxide reduction product), one usually observes one or more spots appearing on the lithium surface from which a red to black colored material is released. In some cases, only a few such spots will appear. More typically, however, large areas of the lithium electrode will be covered with such spots. When the lithium electrode is coupled with a carbon cathode, the open circuit voltage of the resulting electrochemical cell decays rapidly as a consequence of the self-discharge process. This self-discharge represents a major obstacle to the construction of a satisfactory electrochemical cell which comprises an active metal anode, a sulfur dioxide cathode depolarizer, and an electrolyte solution in which the dithionite discharge product is soluble. The prior art fails to disclose any method for either the control or prevention of this self-discharge.
Various alcohols, typically in combination with an inert hydrocarbon diluent, have been utilized to chemically polish and etch alkali metals. A survey of the use of various alcohols to polish lithium, sodium and potassium has been reported by R. N Castellano et al., J. Electrochem. Soc.: Solid State Science, 118, 653 (1971). In addition, it has been reported that the surface of solid lithium can be cleaned by immersion in a mixture of solid CO.sub.2 and methyl alcohol [Aluminum-Lithium Alloys (Proceedings of the First International Aluminum-Lithium Conference sponsored by the TMS-AIME Nonferrous Metals Committee at Stone Mountain, Ga., May 19-21, 1980), T. H. Sanders Jr. and E. A. Starke Jr., Ed., The Metallurgical Society of AIME, p. 20].